Such a control valve is for example disclosed in DE 10 2005 043 458 B4. The shown electrohydraulic control valve comprises an additional changeover valve arranged between a proportional valve and two conduits each of which leads to a pressure chamber. The proportional valve allows to adjust the pressure coming from a high pressure supply and the changeover valve is meant to direct said pressure coming from the proportional valve to one of the pressure chambers. Depending on which pressure chamber is supplied with the higher pressure from the proportional valve the valve element will be moved in one of two directions until an equilibrium between the pressure force and a force by a spring is reached. Each pressure chamber is also connected to one of two two-way magnetic valves, wherein both two-way magnetic valves lead to a low pressure supply.
For a hydraulic valve arrangement it is often required that a control valve cannot just be adjusted manually but also remotely. To this end the valve element is charged with a pressure depending upon the desired direction in which the valve element shall be moved. A pressure may then be maintained until the valve element has reached the desired position.
This functionality can be achieved in several ways.
First of all one may connect both pressure chambers to two magnetic valves each. For each pressure chamber one magnetic valve will be connected to the high pressure supply and one other magnetic valve to the low pressure supply. In order to move the valve element one pressure chamber will then be charged with a higher pressure by opening the corresponding magnetic valve leading to the high pressure supply. At the same time the other pressure chamber will be charged with a lower pressure by opening the corresponding valve leading to the low pressure supply. The two valves stay open until the valve element has reached the desired position.
Such a valve arrangement is reliable but requires four magnetic valves and the corresponding control parts. Consequently the production costs are relatively high.
An alternative way to reach the discussed functionality simply uses only two magnetic valves, each of which is arranged between the high pressure supply and one of the pressure chambers. Each pressure chamber is then connected to the low pressure supply by means of a choke. To displace the valve element one of the magnetic valves is opened. The hydraulic fluid will then flow towards the low pressure supply and charge the choke with a higher pressure. Said higher pressure will then also displace the valve element.
A valve arrangement of this kind requires only two magnetic valves and is thus cheaper in production. On the other hand some amount of hydraulic fluid is lost to displace the valve element and maintaining a non-neutral position of the valve element will usually result in a constant loss of at least some amount of hydraulic fluid.
A further alternative is to use three magnetic valves and one changeover valve as described in DE 10 2005 043 458 B4. Such a valve arrangement requires only three magnetic valves but has the disadvantage that a pressure controlled changeover valve is notoriously difficult to control. For example the pressure difference necessary to make the changeover valve change from one position into the other is usually very small. Furthermore if the proportional valve is opened unintentionally it will usually result in a flow from the high pressure supply across the changeover valve to one of the two-way valves that are open in the normal position. In this case an unwanted deflection of the valve element will result. This makes such a valve arrangement difficult to control and thus undesirable.